Twelve male heart transplant recipients underwent routine electrophysiologic evaluation. None were taking cardioactive drugs, and only two had symptoms of arrhythmia. Two patients had endocardial VVI pacemakers because of previous early sinus node dysfunction. With simultaneous endomyocardial biopsy, we found seven patients with no evidence of rejection (group 1) and five patients with mild rejection (group 2; three initial or mild; two definite rejection). In two group 1 patients with presyncope, corrected sinus node recovery time was prolonged, and pacemakers were implanted into the endocardium. In all patients atrioventricular conduction was normal. One patient had evidence of functional duality of anterograde atrioventricular nodal conduction. In group 2 patients prolongation of effective refractory period of the donor atrium, functional refractory period of the atrioventricular node, and effective refractory period of the right ventricle were observed. This prolongation of refractoriness may be one of the earliest markers of rejection.